1. Field of the Invention
The present invention relates to a tyre manufacturing method and to an apparatus, in particular an extruder, for producing a ribbon in elastomeric material for making at least one component of said tyre.
2. Description of the Related Art
In the present description and in the claims which follow the term “ribbon” means a strip of elastomeric material, the thickness of which is constant or variable in a transverse direction to the strip itself, for use in a rubber article. Preferably said rubber article is a tyre.
It should be pointed out that, for the purposes of the present description, the term “elastomeric material” means a composition comprising at least one elastomeric polymer and at least one reinforcing filler. Said composition preferably also includes additives such as crosslinking agents and/or plasticizers.
A tyre usually comprises: a torically shaped carcass comprising at least one reinforcing ply whose ends are securely attached to a pair of radially inward bead cores, a tread band located around said carcass, and a belt structure between said at least one reinforcing ply and said tread band.
Said belt structure usually comprises at least two strips of rubberized fabric overlaid radially one on top of the other and containing embedded metal reinforcing cords which are laid parallel and side by side within each strip and crossed with those of the adjacent strip, preferably symmetrically with respect to the equatorial plane of the tyre. Preferably, said belt structure also includes a further reinforcing strip radially external to said at least two radially superimposed strips, said further strip containing embedded reinforcing cords laid substantially parallel to the equatorial plane of the tyre.
In tyre manufacturing processes the elastomeric components are, for example, the tread band, the side walls, the airtight rubber layer (liner) covering the internal surface of the tyres designed for use without an inner tube (tubeless tyres), the rubber sheet laid between the tread band and the belt structure or the rubber sheet which sometimes is inserted between two separate belt strips, the sheets of limited width and transversely variable thickness laid underneath the edges of the belt structure, or the sheets of limited width and constant thickness folded around the edges of the belt strips.
Generally, the manufacture of the above-mentioned components requires the initial preparation of an elastomeric compound according to a suitable recipe whose ingredients are chosen to suit the particular component to be obtained.
Successively the preparation of said elastomeric compound, one known process for producing said components involves using an extruder whose function is to work the elastomeric material, as it advances along the extruder body, from a supply section to an extrusion section, forcing the material through a final suitable orifice whose dimensions and geometry are determined according to the profile of the cross section of the component to be obtained.
Usually, extruders are apparatuses that comprise a closed cylindrical body defining in its interior a chamber containing a helical screw for advancing the elastomeric material. Said chamber communicates at one end with the exterior through a hopper (through which the elastomeric compound is introduced) situated in the vicinity of said supply section; while at a second end of said chamber is connected an extrusion head (or extruder head) which communicates with the exterior through the abovementioned orifice positioned in said extrusion section. The walls of the extrusion head usually taper gradually to promote the conveying of the elastomeric material towards said orifice.
The final portion of said helical screw terminates at the beginning of said extrusion head. Said screw has a finned profile running helically along the longitudinal direction of the screw, according to one or more predetermined pitches suitably selected to produce the desired working conditions for the extruder and the rheometric characteristics of the elastomeric compound to be worked.
During its advancing path along the inside of the extruder, due to the rotary movement of the screw, the elastomeric material is subjected to a plasticizing action by the abovementioned finned profile and terminates its path by accumulating in the space between the end portion of the screw and the tapering walls of the extruder head from where the elastomeric material is pushed at high pressure towards the orifice mentioned above.
In the conventional tyre manufacturing technique, each elastomeric component is extruded in its final shape and then sent, e.g. by means of a conveyor belt on which it may optionally be cooled, to winding spools or to storage surfaces (e.g. the so called “book trolley”) ready to be used on the manufacturing machines.
An example of a known extruder for producing tyre components is described in U.S. Pat. No. 5,221,541. Said extruder comprises, downstream of the extrusion screw, a series of metal plates assembled together which together define a longitudinal passage whose cross section is continuously variable and leads into an extrusion orifice. The profile of the cross section of said passage along its longitudinal path varies from a circular section having the greatest area to a substantially oval section having the smallest area. The main characteristic of said longitudinal passage consists in that between the two end sections is present at least one intermediate section whose cross-sectional area is greater than that of the section immediate upstream, the function of said intermediate section being to reduce the temperature of the elastomeric compound prior to its extrusion.
In more recent tyre manufacturing processes, as for example shown in European patent application EP-928,680 in the name of the Applicant, a tyre is formed directly on a toroidal support by overlaying axially adjacent and/or radially superimposed turns of a semi-finished elementary product of appropriate dimensions wound onto said support immediately after it has been made. In particular, three different types of semi-finished elementary products are used, specifically: a profiled strip of elastomeric material only, i.e. the “ribbon” referred to earlier; a strip of elastomeric material containing embedded elongate reinforcing elements, typically textile or metal cords, henceforth defined as “ribbon-like band”; and rubberized metal wires or cords.
The demands made by the most recent tyre manufacturing processes require ribbons of very small cross-sectional dimensions.
The extruders known in the art are capable of producing ribbons of such dimensions provided that their extruder heads are very long in the longitudinal direction. This technological solution, with a gradual change in the cross section of the extrusion channel, avoids the problems of flow instability of the elastomeric material that occur in longitudinally shorter extruders due to sudden change in the cross section of the extrusion channel. Thanks to the longitudinal length of said extruder heads, said technological solution allows easier machining of the inside walls of said channel and prevents the development of shear stresses in the elastomeric material that would generate heat sufficient to cause undesirable scorching of said material.